In recent years, a new two-dimensional carbon material, graphdiyne (GDY), has attracted extensive attention in the field of energy storage, due to its unique topological and electronic structures, high charge mobility, and excellent electron transport properties. However, the disappearance of the density of states near the Fermi level leads to a low quantum capacitance (CQ) of pristine GDY, which limits its application in supercapacitors. Here, we propose doping and metal atom adsorption to be efficient ways to increase the CQ of GDY. Based on first-principles density functional theory, the effects of doping B, N, P, and S atoms and adsorbing Au, Ag, Cu, Ti, and Al atoms on the CQ of GDY are systematically investigated. The results show that the CQ of GDY can be significantly improved by introducing doping/adsorption, which could be a potential cathode material and anode material for supercapacitors. Our work provides an effective way for GDY to be applied as an efficient electrode material for supercapacitors.